EP722850B1 disclosed a low-permeability thermoplastic elastomer composition that is superior as a gas-barrier layer in pneumatic tires. This thermoplastic composition comprises a low-permeability thermoplastic matrix such as polyamides or blends of polyamides, in which a low-permeability rubber such as brominated poly(isobutylene-co-p-methylstyrene), i.e., BIMS, is dispersed. Subsequently, in both EP857761A1 and EP969039A1, viscosity ratio between the thermoplastic matrix and the rubber dispersion was specified as a function of the volume fraction ratio and independently to be close to one in order to achieve phase continuity in thermoplastic and fine rubber dispersions, respectively. Criticality of smaller rubber dispersions was recognized in EP969039A1 in these thermoplastic elastomers for delivering acceptable durability especially for their usage as innerliners in pneumatic tires.
Due to the flow activation and shear thinning characteristic inherent in BIMS polymers, reductions in viscosity values of BIMS polymers at increased temperatures and shear rates encountered during mixing are much more pronounced than reductions in viscosity of the thermoplastic component with which the BIMS polymer is blended. However, minimization of the viscosity differential between the BIMS and thermoplastic components during mixing and/or processing is essential for the provision of uniform mixing and fine blend morphology that are critical for good blend mechanical properties. Considering the lower viscosity of BIMS at high shear rates, either low molecular weight grade Nylons or Nylons with plasticizers are required to match viscosities between BIMS and Nylon for providing vulcanized BIMS and Nylon blends with acceptable dispersion sizes as disclosed in EP857761A1 and EP969039A1. Both options result in the usage of Nylons with compromised mechanical performance and higher permeability.
An alternative method to more closely match the viscosity of the Nylon and BIMS at high shear rates is to enhance the viscosity of the latter component. Relying on the high reactivity of the BrPMS (i.e., brominated p-methylstyrene) group on BIMS, inter-chain associations in BIMS could be introduced through conversion of BrPMS to various strong interacting groups to raise the apparent molecular weights of BIMS and its viscosity. Another method for viscosity enhancement in BIMS is simply to partially crosslink BIMS with some types of curatives (see EP969039A1). However, all methods in increasing the viscosity of BIMS involve reactive conversion of BrPMS group to other groups and, hence, remove it from reactive compatibilization with Nylon in blending and from participation in curing. Therefore, in using a viscosity-enhancement additive for BIMS in thermoplastic resins, which are capable of interacting with bromine, it is critical to select additives that could raise the viscosity of BIMS to that of Nylon with minimal amounts of addition and with minimal, preferably no, effects on reactive compatibilization and cure.
Since the strongest chain associations could be brought upon with either ionic interactions or covalent bonding, a maximum increase in the viscosity of BIMS with a minimum addition of any additive dictates the usage of either tertiary, amines or crosslinkers. Depending on the alkyl chain length and number in tertiary amines, ionic associations could be tailored to raise the viscosity of BIMS to a desirable level with minimal compromises in the properties of BIMS. Among all tertiary amines, dimethyl alkyl amines are most suitable. The alkyl group provides the partial solubility in BIMS, whereas the dimethyl group presents no steric hindrance for ionic associations. Steric restrictive secondary diamines with alkyl end groups are the effective crosslinker for the viscosity enhancement of BIMS. Steric hindrance of the secondary amine in these crosslinkers limits their crosslinking efficiency to prevent gelation, whereas the alkyl end group provides the solubility.